If you've looked at some of the samples, you may have noticed that Objective-S fully supports direct use of URIs in program text. In fact, all identifiers in Objective-S are URIs, resolution of these Polymorphic Identifiers is flexible and to a large-extent user-defined.

A paper introducing Polymorphic Identifiers was presented in October 2013 at the Dynamic Languages Symposium, part of SPLASH/OOPSLA 2013.

The basic idea behind PIs is that identifiers in programming languages designate resources, but what constitutes a resource has expanded dramatically in the last few decades.

  • Programs include a large number of named file resources, especially iOS and Mac OS X apps
  • Many programs nowadays interact with web resources of some sort
  • Many in-program resources have non-standard access requirements
  • Properties attempt to give resource-style interfaces to state implemented by accessors

Schemes and Scheme Handlers

Instead of being built in to the language, access to resource via identifiers is mediated via scheme handlers. Scheme handlers can be either primitive implementing access to a specific kind of resource, or composite combining other scheme handlers in intersting ways (for example implementing a search of several scheme handlers or caching the output of one scheme handler using another).

Once created, scheme handlers are made available to programs by attaching them to a particular scheme, for example http, file or var. Scheme handlers do not have permanent names, but common ones tend to be associated with specific names:

  • var: access to program variables
  • default: scheme that''s used when no scheme is provided by the program. (Usually var:).
  • http: access to web
  • file: access to the file system
  • env: environemnt variables
  • defaults: Cocoa defaults system
  • class: Objective-C/Objective-S classes
  • bundle:, mainbundle: named resources in the class bundle or the main bundle

Composite schemes include the following:

  • relative starts at a path within another scheme handler (common: directory)
  • sequential tries its contained scheme handlers in turns
  • cache similar to sequential, but writes results in later scheme handlers to first scheme handler
  • filter filters the names being passed and/or the data read/written
  • sitemap an in-memory tree of objects

Commonly used schemes are actually composites, for example a user-facing http: scheme typically consists of cache-schemes parametrized with in-memory and relative file schemes in front of the actual raw http: scheme.

Parametrized identifiers

Compound identifiers can include dynamically evaluated components enclosed in curly brackets. The identifier file:{env:HOME}/.bashrc refers to the .bashrc file from the current user''s home directory.


The ref: scheme is special in that it doesn''t indicate a specific domain, but rather stops evaluation at the reference. For example. file:.bashrc retrieves the contents of a the .bashrc, but ref:file:.bashrc returns a reference to that file. References are also useful for parametrizing user interface elements, and generally where one would otherwise be forced to use string keys to refer to parts of objects.

Copyright (c) 2012-2020 Marcel Weiher (st)